1. Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and devices and, more particularly, to mechanisms and techniques for recharging a device that generates a subsea force.
2. Discussion of the Background
During the past years, with the increase in price of fossil fuels, the interest in developing new production fields has dramatically increased. However, the availability of land-based production fields is limited. Thus, the industry has now extended drilling to offshore locations, which appear to hold a vast amount of fossil fuel.
The existing technologies for extracting the fossil fuel from offshore fields may use a system 10 as shown in FIG. 1. More specifically, the system 10 may include a vessel 12 having a reel 14 that supplies power/communication cords 16 to a controller 18. A MUX Reel may be used to transmit power and communication. Some systems have hose reels to transmit fluid under pressure or hard pipe (rigid conduit) to transmit the fluid under pressure or both. Other systems may have a hose with communication or lines (pilot) to supply and operate functions subsea. However, a common feature of these systems is their limited operation depth. The controller 18 is disposed undersea, close to or on the seabed 20. In this respect, it is noted that the elements shown in FIG. 1 are not drawn to scale and no dimensions should be inferred from FIG. 1.
FIG. 1 also shows a wellhead 22 of the subsea well 23 and a drill line 24 that enters the subsea well 23. At the end of the drill line 24 there is a drill (not shown). Various mechanisms, also not shown, are employed to rotate the drill line 24, and implicitly the drill, to extend the subsea well.
However, during normal drilling operation, unexpected events may occur that could damage the well and/or the equipment used for drilling. One such event is the uncontrolled flow of gas, oil or other well fluids from an underground formation into the well. Such event is sometimes referred to as a “kick” or a “blowout” and may occur when formation pressure exceeds the pressure of the column of drilling fluid. This event is unforeseeable and if no measures are taken to prevent it, the well and/or the associated equipment may be damaged.
Another event that may damage the well and/or the associated equipment is a hurricane or an earthquake. Both of these natural phenomena may damage the integrity of the well and the associated equipment. For example, due to the high winds produced by a hurricane at the surface of the sea, the vessel or the rig that powers the undersea equipment may start to drift, resulting in breaking the power/communication cords or other elements that connect the well to the vessel or rig. Other events that may damage the integrity of the well and/or associated equipment are possible as would be appreciated by those skilled in the art.
Thus, a pressure controlling device, for example, a blowout preventer (BOP), might be installed on top of the well to seal the well in case that one of the above events is threatening the integrity of the well. The BOP is conventionally implemented as a valve to prevent the release of pressure either in the annular space between the casing and the drill pipe or in the open hole (i.e., hole with no drill pipe) during drilling or completion operations. FIG. 1 shows BOPs 26 or 28 that are controlled by the controller 18, commonly known as a POD. The blowout preventer controller 18 controls an accumulator 30 to close or open BOPs 26 and 28. More specifically, the controller 18 controls a system of valves for opening and closing the BOPs. Hydraulic fluid, which is used to open and close the valves, is commonly pressurized by equipment on the surface. The pressurized fluid is stored in accumulators on the surface and subsea to operate the BOPs. The fluid stored subsea in accumulators may also be used to autoshear and/or to support acoustic functions when the control of the well is lost. The accumulator 30 may include containers (canisters) that store the hydraulic fluid under pressure and provide the necessary pressure to open and close the BOPs. The pressure from the accumulator 30 is carried by pipe 32 to BOPs 26 and 28.
As understood by those of ordinary skill in the art, in deep-sea drilling, in order to overcome the high hydrostatic pressures generated by the seawater at the depth of operation of the BOPs, the accumulator 30 has to be initially charged to a pressure above the ambient subsea pressure. Typical accumulators are charged with nitrogen but as pre-charge pressures increase, the efficiency of nitrogen decreases which adds additional cost and weight because more accumulators are required subsea to perform the same operation on the surface. For example, a 60-liter (L) accumulator on the surface may have a useable volume of 24 L on the surface but at 3000 m of water depth the usable volume is less than 4 L. To provide that additional pressure deep undersea is expensive, the equipment for providing the high pressure is bulky, as the size of the canisters that are part of the accumulator 30 is large, and the range of operation of the BOPs is limited by the initial pressure difference between the charge pressure and the hydrostatic pressure at the depth of operation.
In this regard, FIG. 2 shows the accumulator 30 connected via valve 34 to a cylinder 36. The cylinder 36 may include a piston (not shown) that moves when a first pressure on one side of the piston is higher than a second pressure on the other side of the piston. The first pressure may be the hydrostatic pressure plus the pressure released by the accumulator 30 while the second pressure may be the hydrostatic pressure. Therefore, the use of pressured canisters to store high-pressure fluids to operate a BOP make the operation of the offshore rig expensive and require the manipulation of large parts.
Still with regard to FIG. 2, the valve 34 may be provided between the accumulator 30 and the cylinder 36 in order to control the timing for applying the supplemental pressure from the accumulator 30. The supplemental pressure may be generated by the accumulator 30, according to an exemplary embodiment, by providing, for example, 16 300-L bottles, each carrying nitrogen under pressure. FIG. 3 shows such a bottle 50 having a first chamber 52 that includes nitrogen under pressure and a second chamber 54, separated by a bladder or piston 56 from the first chamber 52. The second chamber 54 is connected to the pipe 32 and may include hydraulic fluid. When the controller 18 instructs the accumulator 30 to release its pressure, each bottle 50 uses the nitrogen pressure to move the bladder 56 towards the pipe 32 such that the supplemental pressure is provided via pipe 32 to the cylinder 36. The initial pre-charge of the nitrogen is high but as the gas expands its pressure drops. During the operation of a BOP the hydraulic fluid moves a piston on the BOP to close the rams to shear a pipe, casing or other equipment in the wellbore (the term pipe will be used to describe the equipment being sheared). In most cases the pipe in the wellbore is smaller than the bore of the BOP so the initial movement of the ram blocks will not contact the pipe. Once the ram blocks contact the pipe the nitrogen pre-charge in the stored accumulator bottles has expanded substantially so its internal pressure is reduced. This expansion and loss of pressure adversely effect the amount of force available to shear the pipe in the wellbore once the ram blocks finally make contact. Furthermore, the pipe generally collapses before it shears so when the pipe does finally shear the piston has traveled even further which reduces the amount of available pressure to shear the pipe. Once the supplemental pressure in bottle 50 is used, the bottle has to be raised to the surface to be recharged or may be connected via a pipe to the surface such that high pressure is pumped again in the bottle.
Accordingly, it would be desirable to provide systems and methods that avoid the afore-described problems and drawbacks.